W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
The dedicated channel (DCH) for downlink and uplink and the downlink shared channel (DSCH) have been defined in Release 99 and Release 4. In the following years, the developers recognized that for providing multimedia services—or data services in general—high speed asymmetric access had to be implemented. In Release 5 the high-speed downlink packet access (HSDPA) was introduced. The new high-speed downlink shared channel (HS-DSCH) provides downlink high-speed access to the user from the UMTS Radio Access Network (RAN) to the communication terminals, called user equipments in the UMTS specifications.
HSDPA is based on techniques such as fast packet scheduling, adaptive modulation and hybrid ARQ (HARQ) to achieve high throughput, reduce delay and achieve high peak data rates.
Hybrid ARQ Schemes
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which are combined with Forward Error Correction (FEC), called Hybrid ARQ. If Cyclic Redundancy Check (CRC) detects an error, the receiver requests the transmitter to send additional bits or a new data packet. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit will be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ may be defined.
In HARQ Type I the erroneous data packets received, also called PDUs (Packet Data Unit) are discarded and new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU. Using HARQ Type II the erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Finally, HARQ Type III is almost the same packet retransmission scheme as Type II and only differs in that every retransmitted PDU is self-decodable. This implies that the PDU is decodable without the combination with previous PDUs. In case some PDUs are so heavily damaged that almost no information is reusable self decodable packets can be advantageously used.
UMTS Architecture
The high level R99/4/5 architecture of Universal Mobile Telecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, available from http://www.3gpp.org). The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the UEs. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called lu connection with the Core Network (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supports the Serving RNS (S-RNS) 301 by providing radio resources as shown in FIG. 3. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used.
Evolved UTRAN Architecture
Currently, the feasibility study for UTRAN Architecture Evolution from the current R99/4/5 UMTS architecture is ongoing (see 3GPP TSG RAN WG3: “Feasibility Study on the Evolution of the UTRAN Architecture”, available at http://www.3gpp.org). Two general proposals for the evolved architecture (see 3GPP TSG RAN WG3, meeting #36, “Proposed Architecture on UTRAN Evolution”, Tdoc R3-030678 and “Further Clarifications on the Presented Evolved Architecture”, Tdoc R3-030688, available at http://www.3gpp.org) have emerged. The proposal entitled “Further Clarifications on the Presented Evolved Architecture” will be discussed in the following in reference to FIG. 4.
The RNG (Radio Network Gateway) 401 is used for interworking with the conventional RAN, and to act as a mobility anchor point meaning that once an RNG 401 has been selected for the connection, it is retained for the duration of the call. This includes functions both in control plane and user plane.
On the control plane the RNG 401 acts as a signaling gateway between the evolved RAN and the CN, and the evolved RAN and R99/4/5 UTRAN. It has the following main functions:                Iu signaling gateway, i.e. anchor point for the RANAP (Radio Access Network Application Part) connection,                    RANAP connection termination, including:                            Setup and release of the signaling connections                Discrimination of connectionless messages                Processing of RANAP connectionless messages,                                    Relay of idle and connected mode paging message to the relevant NodeB+(s),                        The RNG takes the CN role in inter NodeB+ relocations,        User plane control and        Iur signaling gateway between NodeB+ 402405 and R99/4/5 RNC.        
Further, the RNG is the user plane access point from the CN or conventional RAN to the evolved RAN. It has the following user plane functions:                User plane traffic switching during relocation,        Relaying GTP (GPRS tunneling protocol on the lu interface) packets between NodeB+ and SGSN (Serving GPRS Support Node, an element of the CN) and        Iur interworking for user plane.        
The NodeB+ 402-405 element terminates all the RAN radio protocols (Layer 1—Physical Layer, Layer 2—Medium Access Control and Radio Link Control sub-layers, and Layer 3—Radio Resource Control). NodeB+ 402-405 control plane functions include all the functions related to the control of the connected mode terminals within the evolved RAN. Main functions are:                Control of the UE,        RANAP connection termination,                    Processing of RANAP connection oriented protocol messages                        Control/termination of the RRC (Radio Resource Control) connection and        Control of the initialization of the relevant user plane connections.        
The UE context is removed from the (serving) NodeB+ when the RRC connection is terminated, or when the functionality is relocated to another NodeB+ (serving NodeB+ relocation). Control plane functions include also all the functions for the control and configuration of the resources of the cells of the NodeB+ 402-405, and the allocation of the dedicated resources upon request from the control plane part of the serving NodeB+. The “+” in the term “NodeB+” expresses the enhanced functionality of the base station in comparison to the R99/4/5 specifications.
User plane functions of the NodeB+ 402-405 include the protocol functions of PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) and MAC (Media Access Control) and Macro Diversity Combining.
Enhanced Uplink Dedicated Channel (E-DCH)
Uplink enhancements for Dedicated Transport Channels (DTCH) are currently studied by the 3GPP Technical Specification Group RAN (see 3GPP TR 25.896: “Feasibility Study for Enhanced Uplink for UTRA FDD (Release 6)”, available at http://www.3gpp.org). Since the use of IP-based services become more important, there is an increasing demand to improve the coverage and throughput of the RAN as well as to reduce the delay of the uplink dedicated transport channels. Streaming, interactive and background services could benefit from this enhanced uplink.
One enhancement is the usage of adaptive modulation and coding schemes (AMC) in connection with Node B controlled scheduling, thus an enhancement of the Uu interface. As mentioned in the previous section, in the existing R99/R4/R5 system the uplink maximum data rate control resides in the RNC. By relocating the scheduler in the Node B the latency introduced due to signaling on the interface between RNC and Node B can be reduced and thus the scheduler is able to respond faster to temporal changes in the uplink load. This will reduce the overall latency in communications of the UE with the RAN. Therefore Node B controlled scheduling is capable of better controlling the uplink interference and smoothing the noise rise variance by allocating higher data rates quickly when the uplink load decreases and respectively by restricting the uplink data rates when the uplink load increases. The coverage and cell throughput may be improved by a better control of the uplink interference.
Another technique, which may be considered to reduce the delay on the uplink, is introducing a shorter TTI (Transmission Time Interval) length for the E-DCH compared to other transport channels. A TTI length of 2 ms is currently investigated for use on the E-DCH, while a TTI of 5 ms is commonly used on the other channels. Hybrid ARQ, which was one of the key technologies in HSDPA, is also considered for the enhanced uplink dedicated channel. The hybrid ARQ protocol between Node B and UE allows for rapid retransmissions of erroneously received data units, thus reducing the number of RLC (Radio Link Control) retransmissions and the associated delays. This can improve the quality of service experienced by the end user.
To support enhancements described above, a new MAC sub-layer is introduced which will be called MAC-eu in the following. The entities of this new sub-layer, which will be described in more detail in the following sections, may be located in UE and Node B. On UE side, the MAC-eu performs the new task of multiplexing upper layer data (e.g. MAC-d) data into the new enhanced transport channels and operating HARQ protocol transmitting entities.
E-DCH MAC Architecture at the UE
FIG. 5 shows the exemplary overall E-DCH MAC architecture on UE side. A new MAC functional entity, the MAC-eu 503, is added to the MAC architecture of Rel/99/4/5. The MAC-eu 503 entity is depicted in more detail in FIG. 6 (see 3GPP TSG RAN WG 1, meeting #31: “HARQ Structure”, Tdoc R1-030247, available of http://www.3gpp.org).
There are M different data flows (MAC-d) carrying data packets to be transmitted from UE to Node B. These data flows can have different QoS (Quality of Service), e.g. delay and error requirements, and may require different configuration of HARQ instances. Therefore the data packets can be stored in different Priority Queues. The set of HARQ transmitting and receiving entities, located in UE and Node B respectively will be referred to as HARQ process. The scheduler will consider QoS parameters in allocating HARQ processes to different priority queues. MAC-eu entity receives scheduling information from Node B (network side) via Layer 1 signaling.
E-DCH MAC Architecture at the UTRAN
In soft handover operation the MAC-eu entities in the E-DCH MAC Architecture at the UTRAN side may be distributed across Node B (MAC-eub) and S-RNC (MAC-eur). The scheduler in Node B chooses the active users and performs rate control by determining and signaling a commanded rate, suggested rate or TFC (Transport Format Combination) threshold that limits the active user (UE) to a subset of the TCFS (Transport Format Combination Set) allowed for transmission.
Every MAC-eu entity corresponds to a user (UE). In FIG. 7 the Node B MAC-eu architecture is depicted in more detail. It can be noted that each HARQ Receiver entity is assigned certain amount or area of the soft buffer memory for combining the bits of the packets from outstanding retransmissions. Once a packet is received successfully, it is forwarded to the reordering buffer providing the in-sequence delivery to upper layer. According to the depicted implementation, the reordering buffer resides in S-RNC during soft handover. FIG. 8 the S-RNC MAC-eu architecture which comprises the reordering buffer of the corresponding user (UE) is shown. The number of reordering buffers is equal to the number of data flows in the corresponding MAC-eu entity on UE side. Data and control information is sent from all Node Bs within Active Set to S-RNC during soft handover.
It should be noted that the required soft buffer size depends on the used HARQ scheme, e.g. an HARQ scheme using incremental redundancy (IR) requires more soft buffer than one with chase combining (CC).
E-DCH Signaling
E-DCH associated control signaling required for the operation of a particular scheme consists of uplink and downlink signaling. The signaling depends on uplink enhancements being considered.
In order to enable Node B controlled scheduling (e.g. Node B controlled time and rate scheduling), UE has to send some request message on the uplink for transmitting data to the Node B. The request message may contain status information of a UE e.g. buffer status, power status, channel quality estimate. Based on this information Node B can estimate the noise rise and schedule the UE. With a grant message sent in the downlink from Node B to the UE, Node B assigns the UE the TFCS with maximum data rate and the time intervals, the UE is allowed to send.
In the uplink UE has to signal Node B with a rate indicator message information that is necessary to decode the transmitted packets correctly, e.g. transport block size (TBS), modulation and coding scheme (MCS) level, etc. Furthermore, in case HARQ is used, the UE has to signal HARQ related control information (e.g. Hybrid ARQ process number, HARQ sequence number referred to as New Data Indicator (NDI) for UMTS Rel.5, Redundancy version (RV), Rate matching parameters etc.)
After reception and decoding of transmitted packets on enhanced uplink dedicated channel (E-DCH) the Node B has to inform the UE if transmission was successful by respectively sending ACK/NACK in the downlink.
Mobility Management within R99/4/5 UTRAN
In this section some frequently used terms will be briefly defined and some procedures connected to mobility management will be outlined (see 3GPP TR 21.905: “Vocabulary for 3GPP Specifications” available at http://www.3gpp.org).
A radio link may be a logical association between single UE and a single UTRAN access point. Its physical realization comprises radio bearer transmissions.
A handover may be defined as transfer a user's connection from one radio bearer to another. In a “hard handover” of a new radio link is established. In contrast, during “soft handover” (SHO) radio links are established and abandoned such that the UE always keeps at least one radio link to the UTRAN. Soft handover is specific for networks employing Code Division Multiple Access (CDMA) technology. Handover execution is commonly controlled by S-RNC in mobile radio network.
The “active set” comprises a set of radio links simultaneously involved in a specific communication service between UE and radio network, e.g. during soft handover, the UE's active set comprises all radio links to the RAN's Node Bs serving the UE.
Active set update procedures may be used to modify the active set of the communication between UE and UTRAN. The procedure may comprise three functions: radio link addition, radio link removal and combined radio link addition and removal. The maximum number of simultaneous radio links is commonly set to four. New radio links may be added to the active set once the pilot signal strengths of respective base stations exceed certain threshold relative to the pilot signal of the strongest member within active set. A radio link may be removed from the active set once the pilot signal strength of the respective base station exceeds certain threshold relative to the strongest member of the active set.
The threshold for radio link addition may be typically chosen to be higher than that for the radio link deletion. Hence, addition and removal events form a hysteresis with respect to pilot signal strengths.
Pilot signal measurements are reported to the network (S-RNC) from UE by means of RRC signaling. Before sending measurement results, some filtering is usually performed to average out the fast fading. Typical filtering duration is about 200 ms and it contributes to handover delay (see 3GPP TS 25.133: “Requirements for Support of Radio Resource Management (FDD)”, available at http://www.3gpp.org). Based on measurement results, S-RNC may decide to trigger the execution of one of the functions of active set update procedure (addition/removal of a Node B to/from current Active Set).
E-DCH—Operation During Soft Handover
Supporting soft handover is desirable to obtain the macro diversity gain. In HSDPA for example no soft handover is supported for the HS-DSCH (High Speed Downlink Shared Channel) transport channel. Applying soft handover causes the problem of distributing scheduling responsibilities across all Node Bs of the active set and would require extremely tight timing to provide the scheduling decision to all members of the active set even if distribution of scheduling function were resolved. Only one Node B is transmitting on HS-DSCH to a UE and thus no macro diversity gain is exploited. When UE enters soft handover region for dedicated channels, the Node B, which is allowed to transmit on HS-DSCH, has to be determined. The selection of serving Node B may be done from either the UE side or from network side (by RNC).
In the Fast Cell Selection (FCS) method for HS-DSCH, the UE selects the cell that is the most suitable for transmitting data. UE periodically monitors the channel conditions in the cells within the active set to check whether there is a cell with better channel conditions than the current serving cell.
In case soft handover is not supported for the uplink, a serving Node B has to be selected. One problem, which might occur, is inaccurate selection of the serving Node B. Thus there may be a cell within active set more suitable for uplink transmission than the chosen uplink serving Node B. Therefore, data transmission to a cell controlled by current serving Node B could fail, whereas the transmission to the cells controlled by other Node Bs would have been successful. The accuracy of the selection depends on several factors like signaling delay, filtering of measurement results etc.
To conclude, supporting SHO operation for E-DCH is useful because of macro diversity gain and because possible transmission failures due to an inaccurate selection of the best uplink serving Node B can be eliminated.
Soft Handover Operation without Soft Buffer Synchronization
A flow chart for Node B soft handover operation without soft buffer synchronization assuming R99/R4/R5 architecture is given in FIG. 9. The figure depicts the operation of an arbitrary Node B from the Active Set.
Each Node B within active set monitors the enhanced dedicated physical data channel (E-DPDCH) in step 901 for the reception of uplink traffic. In case a packet is received in step 903 within a transmission time interval (TTI) (see step 902), Node B has to decide if the packet was the initial transmission or a retransmission of a previously sent data packet. The decision is based on associated uplink control signaling, e.g. the New Data Indicator (NDI). In case the received packet was a retransmission then Node B has to combine the received data packet with the previous transmissions stored in the soft buffer before decoding in step 905. For an initial transmission Node B stores (see step 906) the received packet in the corresponding soft buffer (possible previous transmissions stored in the that soft buffer are overwritten) and can immediately try to decode the packet upon reception.
The testing whether decoding was successful or not (see step 907) is done by evaluating the CRC checksum. If the packet is correctly decoded, Node B passes it to higher layer and sends it to S-RNC via lub/lur interface in step 908. In case decoding was not successful the soft information is stored in the soft buffer in step 909.
As outlined above, soft handover operation provides an additional macro diversity gain but also complicates system design to a certain extent. Taking the E-DCH as an example, there is a single transmitting protocol entity and multiple receiving protocol entities for soft handover operation, while for non-soft handover operation there are only a single transmitting and a single receiving protocol entity.
Radio Bearer Establishment
Before starting of any transmission the radio bearer may be established and all layer should be configured accordingly (see 3GPP TS25.331 Radio Resource Control (RRC) protocol specification”, available at http//www.3gpp.org). The procedures for establishing radio bearers may vary according to the relation between the radio bearer and a dedicated transport channel. Depending on the QoS (Quality of Service) parameters, there may or may not be a permanently allocated dedicated channel associated with the RB.
Radio Bearer Establishment with Dedicated Physical Channel Activation
In UMTS the procedure in FIG. 10 may be used when a new physical channel needs to be created for the radio bearer. A Radio Bearer Establishment may be initiated when an RB Establish Request primitive is received from the higher layer Service Access Point on the network side of the RRC layer. This primitive may comprise a bearer reference and QoS parameters. Based on these QoS parameters, Layer 1 and Layer 2 parameters may be chosen by the RRC entity on the network side.
The physical layer processing on the network side my be started with the CPHY-RL-Setup request primitive issued to all applicable Node Bs. If any of the intended recipients is/are unable to provide the service, it may be indicated in the confirmation primitive(s). After setting up Layer 1 including the start of transmission/reception in Node B, the NW-RRC may send a RADIO BEARER SETUP message to its peer entity (acknowledged or unacknowledged transmission optional for the NW). This message may comprise Layer 1, MAC and RLC parameters. After receiving the message, the UE-RRC configures Layer 1 and MAC.
When Layer 1 synchronization is indicated, the UE may send a RADIO BEARER SETUP COMPLETE message in acknowledged-mode back to the network. The NW-RRC may configure MAC and RLC on the network side.
After receiving the confirmation for the RADIO BEARER SETUP COMPLETE, the UE-RRC may create a new RLC entity associated with the new radio bearer. The applicable method of RLC establishment may depend on RLC transfer mode. The RLC connection can be either implicitly established, or explicit signaling can be applied. Finally, an RB Establish Indication primitive may be sent by UE-RRC and an RB Establish Confirmation primitive may be issued by the RNC-RRC.
A simple HARQ operation is currently only possible for a communication between a single transmitter and a single receiver in case of ensuring reliable feedback transmission. The feedback transmission ensures that sender and receiver are synchronized. By increasing the sequence number value of a window based HARQ process or toggling the New Data Indicator (NDI) of a stop-and-wait (SAW) HARQ process in the associated HARQ control information the receiver knows if a new packet is being received and if it can flush the soft buffer accordingly.
This ensures that a new packet will not be combined with a previously stored packet in the receiver. A wrong combining of packets before decoding may be a rare case, but cannot be completely excluded if feedback signaling is not entirely reliable. A correct decoding will not be possible in that case.
Hence the receiver may request for a retransmission of that packet by signaling a NAK. Retransmission of this packet may go on until the maximum number of retransmissions is reached. If there are many retransmissions of a ‘new’ packet which was combined with previous soft buffer values of an ‘old’ packet the influence of the soft values of the ‘old’ packet may be reduced due to successive combining with the new packet allowing a successful decoding of the new packet. How strong the throughput is affected by packet retransmissions may depend on the likelihood of an erroneous operation of the packet retransmission procedure. There may be a trade-off between the overhead spent for reliable signaling and likelihood for erroneous protocol operation. In the same way there may be a procedure to inform the receiver whether a packet has been aborted by the transmitter. This could for instance be caused by reaching the maximum number of retransmissions or in case the assigned delay attribute (or time to live value) could not be met.
Some communication systems as Wideband Code Division Multiple Access (W-CDMA) rely on soft handover operation. In addition to the problem that now multiple feedbacks of each receiver need to be received correctly there is also the problem to synchronize the HARQ soft buffer between the transmitter and a multiplicity of receivers. Not all Node Bs may be able to receive the associated control signaling from the UE, which is needed for a correct processing of the received packet. Assuming that the control information has been received Node B can try to decode the packet and buffer the soft values in case a successful decoding is not possible. It is likely that there is one Node B (e.g. the best link) that is able to decode the packet whereas others do not receive anything.
Transmission of new packets will continue to the best Node B while there are still old packets buffered at other receivers.
In WO 92/37872 a method is introduced that unveils the HARQ operation during soft handover from one transmitter to multiple receivers in the uplink. Reception cannot be guaranteed since power control and thus transmit power is usually adapted to the best link within Active Set. That means as well that reliable feedback from all the receivers is difficult to achieve The transmit power in the uplink needs to be increased for the “bad” links to ensure a well synchronized operation which will increase the uplink interference significantly. WO 92/37872 proposes to increase the HARQ protocol reliability by adding a flush bit to the associated HARQ uplink control information.
A set flush bit informs the receiver not to combine the packet with previous transmissions, but to flush the HARQ soft buffer of that HARQ process. This works in principle, but has two drawbacks. Firstly it assumes that the transmitter knows the state of the receiver, because it has to inform it when to flush the buffer. If the transmitter is not sure about the receiver state due to unreliable or missing feedback the buffer should be flushed. This will lead to loss of information in case the packet had already been received and stored in the soft buffer. Secondly it needs to transmit that flush bit with high reliability along the HARQ control information. This will increase the over the air signaling overhead in the uplink.
The problems of non-synchronized buffers during a soft handover operation with multiple base stations operating as receivers has been described in detail. Existing solutions rely, besides on regular HARQ control information such as HARQ process and HARQ sequence number or NDI, on additional signaling to flush the soft buffer and avoid erroneous combining.